Tube of the Month : The 717A

Hi!

Here is another weird tube which Niclas of Sweden sent so I can feature it as tube of the month: The 717A.

The 717A was developed as a high frequency pentode. But this has not prevented intrepid amplifier designers to use it for audio frequency amplification.

The 717A is a peculiar tube due to its unusual shape which looks like a small mushroom and also resembles the look of a doorknob. This gave it the nickname doorknob tube. It is an indirectly heated pentode and can also be triode wired. The heater runs off 6.3V and only consumes 175mA heater current. The base is 8-pin octal. The base is made of brown micanol and has a metal shell wrapped around it. The tube has seen some use as driver for the 45. But due to its scarcity it has not been used in the audio world very much. The Tung-Sol data sheet lists an equivalent type called 713A which only differs in the base material (bakelite). I did not find any plate curves so I took them myself from a sample. First the pentode curves:

And triode wired, 50V per step on the horizontal axis and 5mA per step vertically. Grid step is 1V:

A nice set of curves! Now lets see some photos of this little gem:

The tube comes in a rather plain box which was typical for military tubes.

Although Western Electric branded and packed the tube is actually manufactured by Raytheon which is also clearly stated on the box:

Some close ups showing some of the details inside:

These must do since I do not own any of these tubes. So no dissection.

And lastly the tube in operation:

In the last photo the grids are nicely visible:

Thanks again to Niclas for lending these tubes to take photos. Stay tuned for more gems from his collection in upcoming Tube of the Month posts.

Best regards

Thomas